2015 Ford F-350 truck with 6.7 V8 Pover Stroke diesel with only couple of thousand kilometers on the odometer came to the shop with concern of Check Engine Light being on, reduced engine power and turbo pressure gauge showing max pressure all the time.

On the test drive I couldn’t verify loss of power or incorrect turbo pressure gauge operation, but engine light was on, indeed. Self-diagnostic test revealed four fault codes stored in PCM memory, one of them confirmed as current fault on KOEO test and one more code was detected after test drive when I run KOER test.

Here is what I got. Code P0405 for EGR Sensor A Circuit Low (this code was stored in memory and also detected during KOEO test). Code P06A7  for Sensor Reference Voltage B Circuit Range / Performance, P0340 for Camshaft Position Sensor A and  P20E9 for diesel exhaust fluid (DEF) pressure too high were all stored in PCM memory. Finally, code P204D for DEF pressure sensor circuit open came up only at KOER test.

It might look like a few different components failed at once. It is not absolutely impossible, but highly unlikely on almost brand new truck. As a matter of fact this list of codes provided me with enough clues to find a problem quickly and easily.

Ford manual says that I have to start diagnostics with code or codes that come up at KOEO test. It works fine in most cases but here I had better idea. With two codes regarding DEF pressure I surely wanted to see first what is a reading from DEF pressure sensor. Also Freeze Frame Data (FFD) was stored for code P20E9 indicating that fault condition that triggered this code was first one detected by PCM (with some exceptions FFD stored only for the first code detected).

While on the test drive I monitored DEF pressure data and noticed it was at 115 psi all the time – this is way too high. After completing KOER test I noticed DEF pressure stays at 8 psi even when engine is not running. Also code P0607 for Sensor Reference Voltage provided another hint indicating there was something that would cause different sensors to give false readings that would be detected by the PCM as fault conditions in multiple systems.

In order to ensure there is no any residual pressure in DEF fluid line I disconnected the line from DEF pump – this way there would be definitely no pressure present. Despite that 8 psi of pressure was still showing in live data monitored with scan tool. Pressure sensor is built into DEF pump assembly and is not replaceable separately but I had couple of faulty DEF pumps handy and could borrow another pressure sensor out of the old pump, just to check. Bingo! With substituted sensor pressure drops to zero indicating that sensor is, indeed, faulty and at the same time ensuring that sensor circuit is OK.

It is a bit of a cheating to check something by replacing a part, spare parts are not always just laying around waiting to be installed just to check if this would fix the problem. Although this saves expensive time so I do that whenever I can.

def pump

This little pressure sensor in DEF pump developed internal short intermittently grounding VREF circuit that supposed to supply stable 5V to a number of different other sensors. With VREF voltage becoming unstable PCM got false readings from numerous sensors and set fault codes for them. Some codes triggered derated power mode of engine operation – this is what was noticed by customer as lack of power. Also incorrect operation of intake pressure sensor caused wrong readings on turbo gauge in the instrument cluster.

Quite simple after all, isn’t it? 🙂